1. Field of the Invention
The present invention relates to an air-fuel ratio control system for internal combustion engines and, more particularly, to an air-fuel ratio control system in which the feed-back control of the air-fuel ratio of a mixture supplied to an internal combustion engine is conducted in accordance with a signal from an air-fuel ratio sensor with a heater. Still more particularly, the invention is concerned with an air-fuel ratio control system of the type mentioned above, which is improved to prevent destruction of the air-fuel ratio sensor when the ambient air temperature is low.
2. Description of the Prior Art
A feed-back type air-fuel control system has been known which employs an air-fuel ratio sensor (oxygen sensor) in which the output is inverted when the air-fuel ratio changes across the stoichiometric point. In order for this type of air-fuel ratio control system to operate satisfactorily, it is essential that the air-fuel ratio sensor is well activated by being heated to and maintained at a high temperature. In some cases, however, this requirement cannot be met particularly when the exhaust gas temperature is comparatively low due to light loads on the engine or when the air-fuel ratio sensor is installed at the downstream portion of the exhaust pipe. In order to obviate this problem, air-fuel control systems have been proposed and actually used in which the air-fuel ratio sensor incorporates an electric heater which heats and activates the sensor.
Air-fuel ratio sensors adapted to produce a digital output which changes linearly in response to a change in the air-fuel ratio have also been put into practical use. Such sensor also incorporate electric heaters for the purpose of improving the sensing accuracy and sufficiently activating the sensors.
Air-fuel ratio control systems using air-fuel ratio sensors of the types mentioned above are broadly used in various internal combustion engines for the purpose of cleaning exhaust gases, regardless of whether the engines are carbureted or fuel injected.
A known air-fuel ratio control system combined with speed-density-type fuel injection and employing an air-fuel ratio sensor adapted to produce a linear output in relation to a change in the air-fuel ratio will be described hereunder by way of example.
The description will be made with reference to FIG. 4 which also will be used in the description of an embodiment of the invention.
In FIG. 4, an internal combustion engine A has an engine proper 1, an intake pipe 2 and a throttle valve 3 disposed in the intake pipe 2.
The pressure of the intake air in the intake pipe 2 is sensed by a pressure sensor 4 which delivers the sensing output to an A/D converter 91 of a later-mentioned control device 9.
The engine speed is sensed by an rpm sensor 5 which produces pulses of a frequency proportional to the engine speed. The output pulses of the rpm sensor are delivered to an input circuit 92 of the control device 9. The control device 9 has an output circuit 96 which delivers a control output in accordance with which a fuel injector 6 operates to inject a fuel into the intake pipe 2.
An air-fuel ratio sensor 8 is disposed in an exhaust pipe 7 which is connected to the engine proper 1. The air-fuel ratio sensor 8 is capable of sensing the air-fuel ratio of the mixture fed to the engine through measurement of components of the exhaust gas flowing in the exhaust gas pipe 7.
Thus, the control device 9 receives various data concerning the state of engine operation, including the intake air pressure data derived from the pressure sensor 4, engine speed data from the rpm sensor 5 and the air-fuel ratio from the air-fuel ratio sensor 8. Upon receipt of these data, the control device 9 computes the optimum fuel injection rate and controls the duty ratio or the pulse width of the driving pulses for driving the fuel injector 6 in accordance with the thus computed optimum fuel injection rate.
The AD converter 91 of the control device 9 is adapted to convert the analog signals such as those derived from the air-fuel ratio sensor 8 and the pressure sensor 4 into digital signals which are delivered to a microprocessor 93.
The input circuit 92 of the control device 9 has a function to conduct a level-conversion of the pulse signal derived from the rpm sensor 5. The signal from this circuit 92 also is delivered to the microprocessor 93. The microprocessor 93 computes the amount of fuel to be supplied to the engine proper 1 in accordance with the digital and pulse signals from the AD converter 91 and the input circuit 92, to produce a signal for controlling the duty ratio or the pulse width of the driving pulses for driving the injector 6.
The processes to be executed by the microprocessor 93 and other related data are stored beforehand in a read-only memory (ROM) 94, while data obtained in the course of computation are temporarily stored in a random access memory (RAM) 95. The delivery of the output signal from the microprocessor 93 to the fuel injector 6 is conducted through the output circuit 96.
The construction of the air-fuel ratio sensor 8 will be described hereunder with reference to FIG. 5. Specifically, the air-fuel ratio sensor 8 has an oxygen pump cell 81, an oxygen battery cell 82, a pair of electrodes 83a, 83b made of a porous material, a diffusion chamber 84, a reference voltage 85, a comparison amplifier 86, a pump driving circuit 87, and a resistor 88 which is used for the purpose of detecting the electric current in the pump cell.
Reference numeral 103 denotes an electrical insulator on which is formed a resistor 100. The resistor 100 serves as a heat-generating element. An air gap 102 is formed between the portion of the electric insulator 103 having the resistor 100 and the oxygen battery cell 82. This basic arrangement of the air-fuel ratio sensor 8 is already known from the disclosures of Japanese patent Laid-Open Nos. 59-19046 and 60-128349. In operation, the voltage generated in the oxygen battery cell 82 and the voltage of a reference voltage source 85 which is set, for example, at 0.4 V are input to the comparison amplifier 86 so as to be compared with each other. At the same time, the pump driving circuit 87 is driven to supply an electric current to the oxygen pump cell 81 so as to reduce the offset of the voltage in the oxygen battery cell 82 from the reference voltage to zero, whereby a state of the exhaust gas corresponding to the stoichiometric ratio is obtained in the diffusion chamber 84.
With this arrangement, it is possible to detect the air-fuel ratio of the mixture which is being fed to the engine, regardless of whether it is on the leaner or richer side of the stoichiometric point, and the result of measurement is taken out as a voltage across the resistor 88. In consequence, an output voltage which linearly changes in relation to a change in the air-fuel ratio over a wide range is obtained as shown in FIG. 5.
During operation of the engine, the resistor 100 is supplied with an electric current through the output circuit 97 in the control device 9 so as to heat and activate the air-fuel ratio sensor 8.
A description will be made hereunder with specific reference to FIG. 7 as to a typical known feed-back control of air-fuel ratio conducted by using the above-described air-fuel ratio sensor 8. FIG. 7 is a flow chart showing the process of the control performed by the control device 9 shown in FIG. 4.
The pulse signal from the rpm sensor 5, representing the rpm Ne of the engine, is read in Step S 1, and the signal from the pressure sensor 4 indicative of the absolute pressure Pb in the intake pipe is read in Step S 2. In Step S 3, the basic driving pulse width r.sub.0 of the pulses for driving the injector 6 is computed on the basis of the data read in Steps S 1 and S 2.
The pulse width r.sub.0 can be expressed by r.sub.0 =K.Pb..eta..sub.v, where K represents a constant, while .eta..sub.v represents charging efficiency which is determined by the intake pressure Pb and the engine rpms Ne.
A target air-fuel ratio (A/F) S is set in Step S 4. The target air-fuel ratio (A/F) S is determined beforehand in such a manner as to optimize the air-fuel ratio for attaining the maximum dynamic performance of the engine while minimizing the fuel consumption under varying engine rpms Ne and the intake pressure Pb, as will be seen from FIG. 8 in which a flow chart (a) shows operation cycle of the engine and in which a flow chart (b) shows the on-off cycle of the heater 100 in the air-fuel ratio sensor 8. The target air-fuel ratio, however, may be determined taking into account also other factors such as the engine temperature and the state of acceleration or deceleration of the engine.
The output signal (A/F) R from the air-fuel ratio sensor 8 is read in Step S 5 and, in Step S 6, the deviation of the air-fuel ratio from the target air-fuel ratio, i.e., (A/F)S (A/F)R, is computed and integrated with a suitable gain. In Step S 7, it is determined whether the integrated value I falls within a predetermined limit range I(LMT). If this integrated value falls within a predetermined range, a correction value I.sub.1 is set as I.sub.1 =I in Step S 8, whereas, if this integrated value does not fall within a predetermined range, a correction value I.sub.1 is set as I.sub.1 =IL in Step S 9.
In Step S 10, the pulse width r of the injector driving pulses is determined by multiplying the basic pulse width r.sub.0 determined in Step S 3 with the correction value I.sub.1 determined in Step S 8 or S 9.
It will be understood that the feed-back control of the air-fuel ratio is conducted to follow the target air-fuel ratio as the above-described control process is repeated momentarily.
The described control operation, however, essentially requires that the air-fuel ratio sensor 8 correctly detect momentary changes in the air-fuel ratio and, therefore, the air-fuel ratio sensor has to be sufficiently activated by being heated. However, exhaust gas temperature is normally so low when the engine is operating under a light load that the air-fuel ratio sensor 8 cannot be sufficiently activated. In order to obviate this problem, it has been a common measure to provide an electric heater 100 in the air-fuel ratio sensor 8 and to supply electric power to the heater 100 whenever the engine is operating, as shown in FIG. 8.
The known air-fuel ratio control system for internal combustion engines described hereinabove can operate satisfactorily under normal ambient air temperature. A problem is encountered, however, particularly when the ambient air temperature is extremely low, e.g., between 0.degree. and -30.degree. C. Namely, under such low ambient air temperatures, if the engine is stopped before the engine and the exhaust system are completely heated, the moisture contained in the exhaust gas condenses within the exhaust pipe 7 to become water droplets which cling to the air-fuel ratio sensor.
The air-fuel ratio sensor 8 has tiny apertures such as the air gap 102 and very small holes formed in the electrodes 83a, 83b. If the engine is left to stand without operating under such cold temperatures, the water droplets clinging to such tiny apertures freeze increasing their volumes to produce mechanical forces which break the cells in the air-fuel ratio sensor 8.